The establishment of a complex body pattern during embryogenesis relies not only on cell determination but on morphogenetic processes involving cell shape changes and movement of epithelial sheets. The process of dorsal closure in Drosophila provides an excellent model system for investigating many aspects of cellular morphogenesis. During dorsal closure, the lateral epithelia on each side of the embryo move dorsally to meet and fuse at the dorsal midline forming a continuous epithelial layer encasing the embryo. The identification of mutants that fail to complete dorsal closure has led to the elaboration of a model in which two signaling pathways, the JUN kinase cassette and the DPP/TGFbeta pathway, cooperate to coordinate initiation and maintenance of epithelial movement. The identity of upstream signals that might activate the JUN pathway and thus the initiation of dorsal closure remain unknown as do the molecular links to the cytoskeleton required for cell shape changes within the epithelium. The goals of this proposal are 1) to gain a better understanding of the morphological and genetic characteristics that contribute to the unique role of cells that make up the leading edge of the dorsal epithelium, 2) to determine whether DPP signaling underlies the polarization and elongation of cells in the lateral epithelium, and 3) to characterize new genes required for the process of dorsal closure, focusing on components that may activate the JUN pathway or that may feed into regulation of the cytoskeleton.